The MIMO Analyzer is a multimode resonant chamber with high Q factor and a very inhomogeneous spatial distribution of the electric and magnetic fields. It consists of two cavities, typically upper and lower, coupled through a metallic plate with slots or a waveguide, which is made accessible by a shielded door. There are various elements and methods to homogenize the fields such as: mode stirrers, slotted metal elements, the movement of the device under test (DUT) inside the chamber or the use of lenses at the slots. Until now all mode stirrers have been proposed to be built on metallic materials, some of them with special forms, as it can be observed from the WO200054365 document. On the other hand, metallic slotted pieces are described on WO2008031964, restricting their application to the aeronautic industry.
Multimode resonant cavities are used in wireless communications applications for laboratory measurements that emulate those made for mobile terminals in Rayleigh propagation scenarios with isotropic distribution of received power. Among the parameters that can be measured there are: diversity gain, MIMO capacity, antenna efficiency, absorbed power, correlation between antennas, specific absorption rate, antenna sensitivity, bit error rate (BER) probability, (these last two have been claimed in U.S. Pat. No. 7,286,961 for reverberation chambers). Thus, until now measurements could only be made for isotropic Rayleigh-fading environments when employing multimode resonant cavities. Furthermore, it is also possible to perform measurements with different dummies filled with lossy fluid with the aim of observing the energy absorption mechanism and its associated reduction in the radiation efficiency of the device under test. This will in turn resemble the behaviour of the device in the presence of a human being, which allows, for example, the investigation on the effects of a user's head in mobile terminals.
The different existing environments for wireless communications can be modelled by using different probability functions. Two of the most common functions used to model these environments are the Rayleigh and Rician functions, with the K factor. The K factor is a parameter that defines the different types of scenarios in Rician environments. These propagating environments determine the performance of wireless communications systems operating within them. As a general rule, macro-cells have greater Rician K factor than micro-cells, that is, the line of sight (LOS) component is more dominant. Moreover, within the cell the K factor decreases with increasing distance to the transmitter. In contrast, the urban environments and those inside buildings often have a rich multipath scattering, which is important enough to make the direct view be hardly dominant, making the statistical distribution of this environment that of a Rayleigh one. Until now only Rayleigh-fading environments could be emulated with multimode resonant cavities, which is a problem. Therefore, when a proper evaluation of terminals is desired for other types of environments, alternative methods such as cumbersome outdoor measurement campaigns in different places are required. These measurement campaigns are costly both time and money wise.
Another application of multimode resonant cavities is microwave heating, drying and curing, that is, these cavities are useful for all processes that involve high power and high frequency electromagnetic fields which are radiated in order to generate heat in a sample of some material. In a microwave oven the object to be heated up is placed either in a fixed position or in a turntable that rotates in order to blend that heat as much as possible. Depending on the incident electromagnetic fields in the sample, the heating will be more or less efficient. In the document ES 2246183 the procedure to obtain the optimal heating position of the sample in terms of electromagnetic fields is described. The objective is to obtain the highest possible efficiency and the highest heating homogeneity as possible. To be able to achieve that goal, the electromagnetic fields have to concentrate uniformly in the sample. There is a problem, however, since heating efficiency can vary from 20% to 90% depending upon the electromagnetic field distribution with the existing methods. When either the size or the properties of the sample material change, the heating efficiency decreases since there is no possibility to change the electromagnetic fields.